Conventional video display monitors employing image interpolation in digital signal processing apply a specific interpolation coefficient for interpolation. Although it is known that a standard function of sinx/x is ideal for deciding an interpolation coefficient, linear interpolation has been adopted in order to simplify hardware configuration.
One conventional technology is explained below with reference to FIG. 7.
A digital video input signal 701, such as a luminance signal, from the outside, changes in accordance with a clock pulse. The digital video input signal 701 is delayed by delay circuits 71 and 72 as set by the clock pulse equivalent to an interpolating picture element. A subtractor 73 subtracts the output signal of the delay circuit 72 from the output signal of the delay circuit 71, and produces the output of the difference in signal levels; or in other words, the difference in signal levels of adjacent picture elements. A multiplier 74 multiplies the above mentioned difference in the signal levels by an interpolation coefficient 702, specifying an interpolation point which is input from the outside, and produces an interpolation signal. An adder 75 adds the output signal of the delay circuit 72 and the output signal of the multiplier 74, and produces a video output signal 703. Since this video output signal 703 is the result of the difference in the signal levels of the adjacent picture elements multiplied by the interpolation coefficient specifying the interpolation point, it can be said that the conventional technology employs linear interpolation.
Using the above system, however, images such as text lose sharpness because they have large difference in signal levels of picture elements. Linear interpolation of such images results in sloped signal edges. If, for example, a step function which is advantageous for interpolating signals for text and the like, is applied to the interpolation coefficient, images of objects such as lamps, for example, acquire jagged edges because they have signal levels with shallow slopes. Moreover, images such as every other vertical or horizontal line comprising high frequency signals may produce a large beat.